MF 165 4WD

Hi There
Can any one help.
I have a mf 165 round axle with a core spline shaft.
I want to make it in to a 4wd but in order to use a 4wd kit i would need to change the main shafts from core spline shaft to a fine spline shaft, if i lets say bout a gearbox off a mf 165 with square axle this would have a fine spline shaft witch would work .
But i would need to change the back end as well Is there any way of changing these shafts, in to a fine spline with out changing the hole back end in other words is it posable to just change the shafts to fine spline?
And is it posable to have fine spline at one end and core spline at the other?
Thanks For Any Help
Rors